As integrated circuit feature sizes decrease, the gate dielectric thickness of field effect transistors (FETs) also decreases. This decrease is driven in part by the demands of overall device scaling. As gate conductor widths decrease, for example, other device dimensions decrease to maintain the proper device scale, and thus device operation. Another factor driving gate dielectric thickness reduction is the increased transistor drain current realized from a reduced gate dielectric thickness. The transistor drain current is proportional to the amount of charge induced in the transistor channel region by the voltage applied to the gate conductor. The amount of charge induced by a given voltage drop across the dielectric is a factor of the capacitance of the gate dielectric.
In order to achieve increased capacitance, gate dielectrics made from oxides such as SiOx are now as thin as 10 Å. These extremely thin gate oxides result in increased gate-to-channel leakage current, however. Problems such as this have led to the use of materials that have dielectric constants that are greater than the dielectric constant of silicon oxide, which has a k value of about 3.9. Higher k values, for example 20 or more, may be obtained with various transition metal oxides. These high-k materials allow high capacitances to be achieved with relatively thick dielectric layers. In this manner, the reliability problems associated with very thin dielectric layers can be avoided while improving transistor performance.
There are, however, fabrication problems associated with forming gate dielectric layers that include high-k materials. For example, processing high-k dielectric layers in the presence of oxygen at elevated temperatures detrimentally affects high-k dielectric films by causing the film to crystallize and also by forming interfacial layers within the high-k film. Another problem associated with the above-mentioned high-k dielectrics is that the forming of a crystalline structure leads to a roughened film surface and unevenly distributed grain boundaries. Surface roughness causes non-uniform electrical fields in the channel region adjacent the dielectric film. Unevenly distributed grain boundaries cause variations in the gate leakage current across the wafer. Such problems degrade MOSFET performance.
For example, damaged dielectric materials may contain a greater number of bulk traps and interface traps than gate dielectrics made from thermally grown SiO2. Traps adversely affect both sub-threshold slope and threshold voltage (Vt). High trap density also leads to leakage through Frenkel-Poole tunneling, and it causes bias temperature instability. Problems relating to flatband voltage (Vfb) shift also occur.
In light of problems such as these, there is a need to develop improved methods and structures involving high-k gate dielectrics and related semiconductor devices that overcome the shortcomings of the prior art.